Shrinkage is one of the most critical properties that control the conversion efficiency of polystyrene foams in wrapping glass and PET containers and in making cups, (including disposal cups). The shrinkage results from molecular orientation induced to the foam during "blown bubble" extrusion process. In this process, the polymer melt with appropriate blowing agents dissolved in it is cooled to the proper extrusion temperature before it is pushed through an annular die. Foaming takes place immediately after the melt exits the die. The foamed bubble is pulled over a sizing mandrel while it is cooled on the inside and outside with air. The combined effect of drawing over the mandrel, low melt temperatures and air cooling results in imparting the shrink property to the foam.
The amount of molecular orientation induced in amorphous polymers depends strongly on the temperature of deformation as well as the strain rate. The most suitable temperature is situated just above the softening point of the material. In practice, however, processability (throughput) dictates the temperature at which orientation is carried out. Many process variables exert strong influence on the orientation imparted to the foam during blown bubble extrusion process. Some of these variable include the temperature of the polymer melt before it exits the die, the line speed, the temperature and amount of air flow used on the inside and outside of the bubble for cooling and the concentration of nucleating and blowing agents.
Many problems have been experienced in controlling the orientation required of PS (polystyrene) foams for converting into disposable cups and in wrapping glass and PET containers. In many cases, the extruded foam with inadequate orientation is unusable. Serious problems have been experienced in labeling PET containers with polystyrene foam labels due mainly to the sensitivity of oriented PET containers to the thermal treatment during labeling operation. Unacceptable bottle shrinkage generally resulted when PET containers were labeled with PS foam labels. In order to eliminate the above problem, polystyrene foams were prepared with halogenated hydrocarbons including halogenated alkanes with at least one fluorine atom such as dichlorodifluromethane as blowing agents and resulted in high residual halogenated hydrocarbon content. Since these halogenated hydrocarbons are good plasticizers for polystyrene, the shrinkage onset temperature was reduced which minimized the bottle shrinkage. The main disadvantage of this approach is that the halogenated hydrocarbon diffuses out of the foam during aging, shipping and handling which increases the shrinkage onset temperature and, therefore, the bottle shrinkage.
One of the disadvantages of foamed plastics for labeling is their poor surface appearance, particularly compared to films. In order to improve the surface appearance for printing and decoration, coextruded foams consisting of polystyrene foam/polystyrene film have been introduced. Because of a very thin solid layer of polystyrene used in the coextruded structure and the interfacial problems associated in coextrusion, the coextruded foams provided a rather rough surface. This rough surface required the use of an undercoat before printing which resulted in increasing the cost of the material. In addition, the coextruded labels could not be used for wrapping PET containers due to differential shrink properties of the foam and solid layers. The conventional method of controlling the orientation during the "blown bubble" extrusion is, therefore, not satisfactory for decreasing the shrinkage onset temperature that is required for labeling PET containers. Lower shrinkage onset temperature is also desirable for increasing the line speed in wrapping glass containers and in converting into disposable cups.
Polystyrene foams have certain deficiencies such as brittleness, easy tearing and easy fracture. Polyethylenefoam/thermoplastic film composites in the prior art were stated to be superior to polystyrene foams in eliminating the brittleness, tearing and easy fracture. As noted in the Rhoades U.S. Pat. No. 4,038,446, assigned to Owens-Illinois, Inc., polyethylene foam/thermoplastic film layers were thought to have superior commercial characteristics. In the Rhoads patent, column 2, lines 31 to 41, it is stated that polystyrene foams have heretobefore been found deficient. The polystyrene foams were found to be possessed of certain deficiencies such as brittleness, easy tearing, and easy fracture. The Rhoads patent leads away from the use of amorphous polystyrene foam and discusses the outstanding characteristics of polyolefin foam such as polyethylene foam for heat shrunk sleeves.
In the prior art, U.S. Pat. No. 4,244,900 to James E. Heider, assigned to Owens-Illinois, Inc., shows the making of a heat shrunk cellular polyethylene sleeve for a container from a two-layer sheet comprising a thermoplastic film and a foamed polyethylene layer.
The Heider patent U.S. Pat. No. 4,244,900 discloses processing of polyolefin foams by secondarily stretching the coextruded polyolefin foam/thermoplastic film sheet at its orientation temperature to provide a heat shrunk sleeve that shrinks at a lower temperature and at a faster rate. The Heider patent further discloses that the thermoplastic foam layer is made of polyethylene or another crystalline polymer.
In the Heider patent, the shrink properties of the crystalline polyethylene foam/crystalline polymer film were improved by stretching the composite after extrusion. It should be noted that polyethylene foam/film composites have not been used generally despite the superior properties stated above. This apparently is attributed to poor shrinkage behavior and dimensional stability problems associated with the above structures.
Since polystyrene foam is the most widely used material for labeling glass and PET containers and for converting into disposable cups, there is a need to improve its surface appearance and the srinkage behavior. It is desirable that the foam shrink at low temperatures while maintaining dimensional stability during shipping and handling.
In the prior art there has been a need for a method of continuously producing a heat shrinkable, one-layer or two-layer sheet that is readily storable without substantial shrinkage at room temperature and a sheet that is readily adapted to heat shrink rapidly around a container such as a bottle to form a label thereon or to form a disposable cup. Hence, a final structure that is a storable, economical, attractive, decorated, single-layer foam sheet or a two-layer sheet comprising a thermoplastic film and a foam layer has been desired.
It is an object of the present invention to provide a method of making a highly oriented single-layer amorphous polystyrene foam sheet or a thermoplastic film/amorphous polystyrene foam sheet with excellent surface smoothness for printing and decoration which can shrink at low temperature to form a label or cup while maintaining dimensional stability at ambient temperature.
It is an object of the present invention to provide an outstanding method of continuously producing a heat shrinkable thermoplastic film and an amorphous polystyrene foam to form a heat shrinkable two-layer sheet that is easily storable without substantial shrinkage at ambient temperature and a sheet that will heat shrink rapidly to form a label or a cup, the method comprising the steps of:
A. coextruding a thermoplastic film and an amorphous polystyrene foam layer to form a two-layer sheet having a continuously formed length as it is conducted away from the coextruder, the length being greater than the width of the sheet;
B. bringing the two-layer sheet to its orientation temperature after the foam layer is formed;
C. stretching the two-layer sheet at its orientation temperature in a longitudinal direction to reduce its thickness and to provide a dimensionally stable, storable, oriented thermoplastic film/amorphous polystyrene foam sheet;
D. printing on the oriented sheet to decorate the same to thereby provide an oriented, decorated, storable sheet; and
E. heat shrinking the storable, oriented sheet to form a final attractive, decorated structure.